Depending upon their density, lipoproteins in living bodies are classified into high-density lipoprotein, low-density lipoprotein (hereinafter, abbreviated as LDL), very low-density lipoprotein (hereinafter, abbreviated as VLDL) and chylomicron (hereinafter, abbreviated as CM). Their functions in living body greatly differ mostly depending upon the difference in the kind of apoprotein and their lipid compositions vary as well. Among them, HDL relates to removal of cholesterol accumulated in cells by receiving cholesterol from various tissues including arterial wall and is a factor for the prevention of risk of various arteriosclerotic diseases such as coronary arteriosclerosis. HDL level in blood has been known to be useful for predicting the onset of arteriosclerotic diseases.
The conventional method for quantitatively determining cholesterol in HDL (hereinafter, abbreviated as HDL cholesterol) comprises two steps; a step of fractionation by an ultracentrifugal method, an immunochemical method, an electrophoretic method, a precipitation method, and the like; and a step of quantitative determination of cholesterol. However, the step of fractionation is complicated and takes a long time to operate, and moreover, there is a problem in terms of safely. Therefore, the measuring methods comprising such step of fractionation is not suitable for practical use because it is too inefficient.
In recent years, various measuring methods have been reported for solving the above problems. For example, there have been known, for example, a methods comprising mixing serum or plasma in a buffer comprising cholesterol esterase and cholesterol oxidase, and a salt of bile acid, a bile acid derivative or dioctyl sulfosuccinate, with the enzymes to react cholesterol in VLDL and LDL prior to HDL cholesterol, measuring the formed hydrogen peroxide, adding a nonionic surfactant having polyoxyethylene oxide group to the reaction solution to react HDL cholesterol with the enzymes and measuring HDL cholesterol fractionately (Japanese Published Unexamined Patent Application No. 69999/1987); and a measuring method for quantitatively determining HDL cholesterol comprising reacting serum, in a buffer comprising pancreas-derived cholesterol esterase, cholesterol oxidase, a bile acid-type surfactant and a nonionic surfactant, with the enzymes at a specific pH and specific temperature (Japanese Published Unexamined Patent Application No. 126498/1988). In a measuring method mentioned in the document 2, as the reaction of LDL cholesterol with the enzymes firstly proceeds and then reaction of HDL cholesterol with the enzymes proceeds, the determine HDL cholesterol can be done. However, those methods take a long time to operation and, moreover, they are not always specific to HDL cholesterol.
With regard to a method for quantitatively determining HDL cholesterol where lipoproteins other than HDL are aggregated, there have been known, for example, a measuring method using a reagent which aggregates lipoproteins other than HDL such as dextran sulfate, a divalent metal salt and a chemically modified enzyme (Japanese Published Unexamined Patent Application No. 131197/1996), a measuring method using a reagent which forms a complex with lipoproteins other than HDL such as polyanion and a surfactant which does not dissolve lipoprotein such as a polyoxyethylene-polyoxypropylene copolymer (Japanese Published Unexamined Patent Application No. 201393/1996), a measuring method using polyanion such as dextran sulfate, a divalent metal salt, a specific nonionic surfactant and albumin which is supplemented to the albumin contained in the sample (Japanese Published Unexamined Patent Application No. 285298/1997); a method for measuring HDL cholesterol in serum or plasma comprising treating serum or plasma with a solution comprising a lipoprotein fractionating agent (a combination of polyanion such as dextran sulfate with divalent cation such as magnesium ion), without separating the resulting mixture solution into solid and liquid, treating the solution with cholesterol esterase and cholesterol oxidase in the presence of anionic surfactant (alkyl sulfonate, bile acid or derivatives thereof), and measuring the formed hydrogen peroxide (Japanese Published Unexamined Patent Application No. 116996/1996), a method comprising adding a substance which forms a complex with lipoproteins other than HDL such as polyanion and a specific surfactant which does not dissolve lipoprotein to a biological specimen and measuring HDL cholesterol enzymatically (Japanese Published Unexamined Patent Application No. 201393/1996), and the like.
In those methods for quantitatively determining HDL cholesterol where lipoproteins other than HDL are aggregated, they have a good correlation to the conventional standard method. However, there are problems such as inaccuracy due to turbidity by aggregates formed in the reaction and an excessive load to autoanalyzer due to deposition of metal hydroxide produced in reaction cells by the reaction of metal salt in the reaction solution with an alkali used for washing of reaction cells.
With regard to a method for quantitatively determining HDL cholesterol without aggregation of lipoproteins other than HDL, there have been known, for example, a measuring method of HDL cholesterol comprising contacting a biological specimen with cholesterol esterase derived from pancreas and cholesterol oxidase in the presence of bile acid or a salt thereof and albumin, and measuring the compound which is consumed or produced by the enzymatic reaction of HDL cholesterol (International Publication WO 97/40376); a measuring method of HDL cholesterol in a sample comprising reacting a sample with cholesterol esterase and/or lipoprotein lipase acting on HDL fraction preferentially and cholesterol oxidase in the presence of a nonionic surfactant which has HLB value more than 16 as well as a reaction selectivity to the HDL fraction (International Publication WO 00/52840), and the like. In addition, a method where cholesterol in lipoproteins other than HDL is selectively converted to hydrogen peroxide using acylpolyoxyethylene sorbitan ester, the resulting hydrogen peroxide is eliminated and HDL cholesterol is enzymatically measured by adding polyoxyethylene alkyl ether (Japanese Published Unexamined Patent Application No. 299/1997) has been known as well.
However, in those methods for quantitatively determining HDL cholesterol where lipoproteins other than HDL are not aggregated, there may be a problem of inaccuracy of measured value caused by an incomplete elimination of cholesterol in lipoproteins other than HDL or by a non-specific reaction with cholesterol in lipoproteins other than HDL.
Further, when a specific substance in a sample is measured by an optical means, it has been a serious problem that turbidity caused by water-insoluble protein gives an optical effect to the determination in the case of sample which are derived from patients suffering from M proteinemia, myeloma and the like. In order to avid the optical effect, there has been commonly known a method that solves the turbidity caused by water-soluble protein by adjusting salt in the reaction solution at high concentration. However, in the quantitative determination of HDL cholesterol, there are some cases that presence of salt at high concentration may induce a decrease in specificity. Therefore, the quantitative determination of HDL cholesterol in the presence of a salt at high concentration is often very difficult.